Merkel, who stood down on Wednesday after 16 years in power, “does not want to retell her entire life,” Baumann said in an interview with Der Spiegel magazine.
She wants to explain her key political decisions in her own words, and look back on her life’s journey,” she said.
The book will be a joint project between Merkel and Baumann, a key adviser at Merkel’s side from the beginning of the 1990s right up to her retirement this week.
The notoriously discreet Baumann wrote some of Merkel’s most important speeches and also took part in strategic decision-making.
According to Baumann, the project will take two to three years and there are no plans for support from external authors.
“The chancellor and I were quite certain: if we were going to do this book, we would do it alone – without ghost writers, without historians, without journalists,” Baumann said.
Merkel, 67, has been tight-lipped about her future plans after handing over power to Social Democrat Olaf Scholz on Wednesday.
“I want to think carefully about what I want to do in the next phase of my life,” she told an audience earlier this year during a joint interview with Nigerian writer Chimamanda Ngozi Adichie.
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“Do I want to write, do I want to speak, do I want to go hiking, do I want to stay at home, do I want to see the world? I’ve decided to just do nothing to begin with and see what happens.”